The present invention relates to sensors capable of measuring forces along three translational axes and measuring moments about each of these three axes. More particularly, the present invention relates to force sensors having three or four legs, each of which contain two shear strain gages oriented to sense forces in directions orthogonal to one another.
For many applications, it is important to accurately and dynamically determine forces acting on a body such as a machine tool at the end of a robotic arm. The magnitude and direction of such forces may be described in terms of three mutually orthogonal axes (e.g., the conventional X, Y, and Z Cartesian coordinates) and the moments about each of these axes. Various force sensors are employed for such tasks. One widely-used type of force sensor is the strain-based force sensor. These sensors generally include a body that deforms under applied forces and moments, and some transducer that is sensitive to the strain in the body. The transducer, often a strain gage (foil or semiconductor), is placed on the body in a location of expected strain. Usually, the strain gage is placed in a location and at an orientation in which one form of strain is dominant - bending, shear, or extensional strain. By measuring the strains in appropriate locations, one can then calculate the forces and moments applied to the body.
Many strain-based force sensors are known in the art. However, the available sensors suffer from various shortcomings. For example, many force sensors measure "bending strain" which requires a relatively large deformation in order to generate a sufficiently large output (in comparison to the amount of deformation required to generate the same output in a shear or elongation detecting sensor). The bending referred to here is a deformation of a beam or arm that is initially planar and bends so that it is no longer planar. To generate a sufficiently large bending strain, the arm on which a strain gage is mounted must be relatively flexible. Unfortunately, this can result in signal degradation by lowering the natural frequency of the sensor, limiting frequency response, and causing vibrations in the overall system. Further, the relatively large displacements required for bending strain measurements can take the sensor out of a linear response regime (i.e., the strain of the body is no longer directly proportional to the applied force), thus complicating interpretation of strain gage interpretation of strain gage outputs. Still further, if the sensor arms are made too thin, the entire sensor may fail prematurely when relatively large forces are applied.
Another issue arises with force sensors which have multiple strain gages mounted on the same arm. Often the strain gages provided on such sensors require different levels of sensitivity. Unfortunately, the sensitivity to forces in one direction is often coupled to the sensitivity of forces in an orthogonal direction. Thus, it may be impossible to increase the sensitivity of one strain gage mounted on a sensor arm without also increasing the sensitivity of another strain gage mounted on the same ann. This can be understood by considering one method for increasing sensitivity: decreasing the thickness of the arm on which the strain gages are mounted to produce a more flexible arm. Consider, for example, U.S. Pat. 4,094,192 which uses beams that have shear strain gages to measure forces acting perpendicular to the sensor axis, and extensional gages to measure forces along the sensor axis. To increase the sensitivity of the extensional gages to bending forces, the cross sectional area of the beam can be reduced. However, this reduction in area will also affect the strain measured by the shear gages. Thus, the sensor can not be engineered so that axial force sensitivity can be adjusted independently of the lateral force sensitivity.
Another problem associated with many strain gages is the potentially large amount of effort and expense associated with their manufacture. In some designs, the sensor body takes on complicated and difficult to machine shapes such as combinations of beams extending radially from cylindrical shells. In addition, some designs require multiple strain gages placed at various hard to reach locations on the sensor body. For example, some force sensors have strain gages mounted on both the inside and outside surfaces of hollow cylindrical sensor bodies. See e.g., U.S. Pat. Nos. 4,640,138 and 4,823,618. The inside surfaces of such bodies can be especially difficult to reach. Furthermore, some designs require many gages to be placed within a small area (see U.S. Pat. No. 4,911,024). Accurate placement of the strain gages in such sensors must often be done manually - a time consuming and expensive task.
Thus, there exists a need for improved force sensors which are relatively easy to manufacture, do not rely on bending strain, and have decoupled sensitivity to forces in orthogonal directions.